In MIMO system, access point and user station use multiple antennas to obtain the higher rate by the method of spatial multiplexing. Compared with the general method of spatial multiplexing, an enhanced technology is that the user station feeds back Channel State Information (CSI) to the access point, and the access point uses some transmission pre-coding technologies based on the obtained CSI, thus to improve the transmission performance.
There are many methods for obtaining Channel State Information in MIMO systems, generally using the CSI feedback technology. IEEE 802.11n proposes a scheme for providing a quantized feedback CSI matrix, in which an access point initiates the feedback request and a user station gives feedback of sub-carrier matrix Heff on the quantized MIMO channel. The access point calculates the pre-coding matrix Qk based on the sub-carrier matrix Heff. The matrix Heff of CSI includes the equivalent channel between the input from the space mapping of transmitting terminal and the output from the FFT of receiving terminal. In order to facilitate the description of the quantized feedback process, in the following disclosure, a user station is also referred to as a transmitting terminal, and an access point as a receiving terminal.
The specific method of realizing quantized feedback is shown in FIG. 1. In Step S101, the method includes calculating the maximum value of a real part and an imaginary part of each element of CSI matrix Heffq(k) of the sub-carrier by the transmitting terminal:
                                          m            H                    ⁡                      (            k            )                          =                  max          ⁢                      {                                          max                ⁢                                  {                                                                                                          Re                        ⁡                                                  (                                                                                    H                                                              eff                                ⁡                                                                  (                                                                      m                                    ,                                    l                                                                    )                                                                                                                      ⁡                                                          (                              k                              )                                                                                )                                                                                                                                                          m                        =                        1                                            ,                                              l                        =                        1                                                                                                            m                        =                                                  N                          r                                                                    ,                                              l                        =                                                  N                          c                                                                                                      }                                            ,                              max                ⁢                                  {                                                                                                          Im                        ⁡                                                  (                                                                                    H                                                              eff                                ⁡                                                                  (                                                                      m                                    ,                                    l                                                                    )                                                                                                                      ⁡                                                          (                              k                              )                                                                                )                                                                                                                                                          m                        =                        1                                            ,                                              l                        =                        1                                                                                                            m                        =                                                  N                          r                                                                    ,                                              l                        =                                                  N                          c                                                                                                      }                                                      }                                              (        1        )            wherein Heff(m,l)(k) refers to an element in Heff(k); Re(Heff(m,l)(k)) refers to the real part of Heff(m,l)(k); Im(Heff(m,l)(k)) refers to the imaginary part of Heff(m,l)(k); m is a line position parameter; l is a column position parameter; Nr is the maximum line number; Nc is the maximum column number; 1≦m≦Nr, 1≦l≦Nc, Nr≧1, Nc≧1 m, l, Nr and Nc are positive integers; and k is a position parameter of the sub-carrier, which may be a serial number.
In Step S102, the method includes carrying out 3-bit quantization to the relative value
            max      ⁢                        {                                    m              H                        ⁡                          (              z              )                                }                          z          =                      -                          N              SR                                                z          =                      N            SR                                              m        H            ⁡              (        k        )              ⁢          ⁢  of  ⁢          ⁢            m      H        ⁡          (      k      )      by said transmitting terminal to obtain the quantization result MH(k)
                                          M            H                    ⁡                      (            k            )                          =                  min          ⁢                      {                          7              ,                              ⌊                                  20                  ⁢                                      log                    10                                          (                                                                        ma                          ⁢                                                                                                          ⁢                          x                          ⁢                                                                                    {                                                                                                m                                  H                                                                ⁡                                                                  (                                  z                                  )                                                                                            }                                                                                      z                              =                                                              -                                                                  N                                  SR                                                                                                                                                    z                              =                                                              N                                SR                                                                                                                                                                                          m                            H                                                    ⁡                                                      (                            k                            )                                                                                              )                                                                      ⌋                                      }                                              (        2        )            wherein max{mH(z)}z=NSRz=SR is the maximum amplitude value Alpha, └x┘ is the maximum integer not exceeding x; and NSR is the subscript of the maximum data sub-carrier.
In Step S103, the method includes calculating the linear portion MHlin(k) of MH(k) of said transmitting terminal:
                                          M            H            lin                    ⁡                      (            k            )                          =                              max            ⁢                                          {                                                      m                    H                                    ⁡                                      (                    z                    )                                                  }                                            z                =                                  -                                      N                    SR                                                                              z                =                                  N                  SR                                                                          10                                                            M                  H                                ⁡                                  (                  k                  )                                            /              20                                                          (        3        )            
In Step S104, the method includes carrying out Nb bit quantization to the real part and imaginary part of each element in Heff(k) matrix respectively by said transmitting terminal:
                              H                      eff            ⁡                          (                              m                ,                l                            )                                            q            ⁡                          (              R              )                                      =                  round          ⁡                      (                                                            Re                  ⁡                                      (                                                                  H                                                  eff                          ⁡                                                      (                                                          m                              ,                              l                                                        )                                                                                              ⁡                                              (                        k                        )                                                              )                                                                                        M                    H                    lin                                    ⁡                                      (                    k                    )                                                              ⁢                              (                                                      2                                                                  N                        b                                            -                      1                                                        -                  1                                )                                      )                                              (        4        )                                                      H                          eff              ⁡                              (                                  m                  ,                  l                                )                                                    q              ⁡                              (                I                )                                              ⁡                      (            k            )                          =                  round          ⁡                      (                                                            Im                  ⁡                                      (                                                                  H                                                  eff                          ⁡                                                      (                                                          m                              ,                              l                                                        )                                                                                              ⁡                                              (                        k                        )                                                              )                                                                                        M                    H                    lin                                    ⁡                                      (                    k                    )                                                              ⁢                              (                                                      2                                                                  N                        b                                            -                      1                                                        -                  1                                )                                      )                                              (        5        )            
In Step S105, the method includes feeding back Alpha, MH(k) and quantized Heffq(k) to the receiving terminal by said transmitting terminal. In Step S106, the method includes receiving Alpha, MH(k) and quantized Heffq(k) by said receiving terminal.
In Step S107, the method includes calculating the linear value according to MH(k) by said receiving terminal as follows:r(k)=10MH(k)/20  (6)
In Step S108, the method includes scaling the real part Heff(m,l)q(R)(k) and imaginary part Heff(m,l)q(I)(k) of each element Heff(m,l)q(k) in Heffq(k) according to Alpha and r(k) by said receiving terminal, thus to recover the CSI matrix (also known as H matrix):
                                          Re            ⁢                          {                                                                    H                    ~                                                        eff                    ⁡                                          (                                              m                        ,                        l                                            )                                                                      ⁡                                  (                  k                  )                                            }                                =                                    max              ⁢                                                {                                                            m                      H                                        ⁡                                          (                      z                      )                                                        }                                                  z                  =                                      -                                          N                      SR                                                                                        z                  =                                      N                    SR                                                              ⁢                                                H                                      ff                    ⁡                                          (                                              m                        ,                        l                                            )                                                                            q                    ⁡                                          (                      R                      )                                                                      ⁡                                  (                  k                  )                                                                                    r                ⁡                                  (                  k                  )                                            ⁢                              (                                                      2                                                                  N                        b                                            -                      1                                                        -                  1                                )                                                    ⁢                                  ⁢                              Im            ⁢                          {                                                                    H                    ~                                                        eff                    ⁡                                          (                                              m                        ,                        l                                            )                                                                      ⁡                                  (                  k                  )                                            }                                =                                    max              ⁢                                                {                                                            m                      H                                        ⁡                                          (                      z                      )                                                        }                                                  z                  =                                      -                                          N                      SR                                                                                        z                  =                                      N                    SR                                                              ⁢                                                H                                      ff                    ⁡                                          (                                              m                        ,                        l                                            )                                                                            q                    ⁡                                          (                      I                      )                                                                      ⁡                                  (                  k                  )                                                                                    r                ⁡                                  (                  k                  )                                            ⁢                              (                                                      2                                                                  N                        b                                            -                      1                                                        -                  1                                )                                                                        (        7        )            
Through the de-coding process (formula 7) of quantized CSI matrix from the receiving terminal, it can be determined that the feedback overhead required under the method of CSI matrix quantized feedback is the sum of the required bit number of Alpha, MH(k) and the quantized Heffq(k). NAlpha+3+2×Nb×Nr×Nc.